I was unable to learn R because I couldn't understand what a dataframe is. It was irritating that it wasn't defined clearly and there seemed to be no connection to terminology that was familiar to me (relational databases, SQL tables etc.).
There's no other way to say this without sounding rude, but you weren't unable to learn R because you couldn't understand what a dataframe was - you were unable to learn R because you gave up. Blaming a data structure for the failure seems like a bit of a stretch.
That is what I said. I didn't understand what a dataframe was. Note that I didn't really blame the data structure. I said it was an irritating factor. Someone else might not have been as irritated. But if R was closer to what I already knew, learning would have been easier. For me.
In R a dataframe is just a named list of vectors, where each vector has equal length.